How were the first women's uniforms designed for modern militaries?

Interesting question!

Let's start with your last question, since that's actually fairly straightforward to answer.

Why do so many uniforms for women include skirts, which are a highly gendered piece of clothing and kinda impractical to boot?

You basically answered your own question right there... Skirts are a highly gendered piece of clothing, and they were consciously chosen for women's uniforms because, even though women were being allowed to serve in the military, they were still considered apart from the traditional masculine military narrative.

As M. Michaela Hampf puts it:

Despite the successful institutional integration of the women’s corps into the U.S. Army, the women remained the constitutive outside of the organization. The uniforms with their symbolic as well as material aspects highlight the contradictions in the process of integrating women into the military. Military and societal discourses on women and soldiers were materialized in the design of the women’s uniforms. Likewise, the way the uniforms were procured and distributed by the army reflects the reluctance with which the Army regarded the women’s Corps as a military institution. This brings the power structures into view, within which the construction of the category woman/soldier took place. At the same time, the construction of the woman/soldier was determined by the use(s) the women themselves made of the uniform. (Release a Man for Combat: The Women's Army Corps During World War II)

Hampf then goes on to say:

The symbolic character is highlighted by the choice and design of the uniform and insignia during the initial period of the WAAC. During the planning phase questions of how “feminine” and how “soldier like” Waacs were supposed to look in order to comply with both societal expectations and military expediency were negotiated of the design of the uniform. The enormous amount of advertising for the uniform was thought necessary in order to convey the notion of a “respectable femininity” further emphasizes the symbolic significance. The constant changes in the appearance of the uniform during the war also express the importance and fragility of this concept. (Ibid.)

As for how the first women's uniforms were developed (again, this is from a United States military perspective-- Anyone with info on non-U.S. military uniforms for women please chime in), it's like anything with the military... It's design by committee. In fact, other than the SS having Hugo Boss on the payroll designing their (admittedly pretty snazzy) uniforms, American military uniforms were put together by a senior group of military leaders, and women's uniforms were apparently no different. Hampf goes into a lot of detail talking about how the Army's Quartermaster Corps, which had designed men's uniforms for over 200 years by the time WWII rolled around, had a hell of a time figuring out how to strike the societally necessary balance between "ladylike" and "utilitarian." It was so inconvenient for women enlistees that there were complaints constantly about the skirted uniforms being impractical for the jobs that needed to be done. Hampf outlines the response from the War Department to these issues in a sadly hilarious little missive known as "Pamphlet 35-2: The WAC Officer -- A Guide To Successful Leadership":

The woman in uniform in the military setting is often doing a man’s work. Yet she must constantly remember that her effectiveness is going to be decreased if she tries to imitate the man or if she tries to trade on her sex. She must remain feminine in her personality; be military in the performance of her duty. She should not be afraid to accept gracefully the courtesies which American men naturally accord to women.

^I ^will ^refrain ^from ^making ^any ^editorial ^comment ^here.

Anyway, the book I'm pulling all of this from has a wealth of info about the creation of the women's Army corps in WWII, and if you're interested in this topic, I definitely recommend it.

Another journal that looks good is A Companion to Women's Military History, but it's admittedly covering a broader range of military history than just the 20th century. There's also Fighting Different Wars: Experience, Memory, and the First World War in Britain, which has a chapter dedicated to men and women's service in the preceding war to WWII.

Edited for spelling and formatting